Self-tuning klystron



AUS-25, 1953 M. GARBUNY 2,650,324 A SELFHTUNING KLYSTRON vFilm1 Jan. 19,1949 ATTORNEY Patented Aug. 25, 1953 `SELF-TUNING KLYSTRON Max Garbuny,Pittsburgh, Pa., assignor to Westinghouse Electric Corporation,

East Pittsburgh,

Pa., a corporation of Pennsylvania Application January 19, 1949, SerialNo. 71,598

(Cl. S- 6) 4 Claims. 1

My invention relates to oscillation generators, and more particularly tovelocity-modulated oscillation generators employing self-tuning resonantcavities.

In the prior art devices of which I am aware, considerable di'iculty hasbeen encountered in tuning two cavity velocity-modulated tubes, such asthe klystron, to a desired frequency. In an effort to overcome this, thereflex klystron lhas been adopted, but this device has the disadvantageof a high power loss. i

It is, accordingly, `an object of my invention to provide a two cavityvelocity-modulated tube of high power which is easily tuned.

Another object of my invention isto provid a two cavityvelocity-modulated tube in which one cavity is automatically self-tuningin response to the oscillations received from the other cavity. l

An ancillary object of my invention is to provide a two cavityvelocity-modulated amplifier, the resonant frequency of which will varyin response to the input oscillation. l

In accordance with the present invention, I provide a two cavityvelocity-modulated tube similar to the klystron. In each of the cavitiesis placed an adjustable probe for varying the frequency of the cavity,the depth of which may be varied manually. In one of the cavities,preferably the one nearest the anode, is placed a small responsive capor probe attached by a spring to the body ofthe klystron. `Thisresponsive probe has been described in `detail in my copendingapplication Serial No. 65,963, filed December 17, 1948...

In accordance with one modification of my invention the responsive probeis so oriented with respect to the electric field generated in thecavity that itA may have electric forces of attraction supplied theretohaving a magnitude proportional to the square of the magnitude of theelectric field. The responsive probe may be subjected to the reaction ofa restoring force which opposes the force generated by the electricfield. The probe then assumes a position such that the force generatedby the electric field and the restoring force are equal and opposite forany frequency at which the` cavity may be driven. A change in drivingfrequency from that of the resonance equilibrium will cause an unbalanceof the electric and restoring forces. This unbalance is, in turn, usedto change the position of the responsive probe in such a way as torestore the balance of forces and hence produce resonance at the changeddriving frequency.

In a similar way, a constant power level is maintained inside thecavity. For a given probe position the electric force on it is sharplydependent on frequency in the resonance region. As long as the drivingoscillations remain constant not only in frequency but also in power,the probe remains in a constant position. Should the magnitude of thedriving signal vary, however, without changing frequency, the magnitudeof the force generated within the cavity will likewise change, and theresponse of the tuning element to the change, will be to cause a smallchange in resonance frequency of the cavity such that the amplitude ofthe signals within the cavity will be maintained at a constant level.Thus, the cavity maintains a constant power level when the drivingsignal is constant independent of signal power deviations.

Likewise, should `both the frequency and amplitude of the driving signalvary simultaneously from an original value, the resonant frequency ofthe cavity will be modified to such a value that the force applied tothe probe by the electric field within the cavity will preciselycounterbalance the restoring force applied to the probe. Thus theresonant frequency of the cavity will be maintained at such a value thatthe power level of the cavity at the driving frequency remains constant.

By employing this self-tuning cavity. the diiiiculty encountered intuning two cavity klystrons of high power to adesired frequency can besubstantially `avoided. Once the tube is in operation, the frequency ofthe tube may be changed simply by adjusting the tuning probe in theknown self-tuning cavity. This will change the frequency of themodulations entering the selftuning cavity. The change in frequency willcause a change in the force on the self-tuning cavity causing it to moveand compensate for the change in frequency.` Thus, it is only necessaryto tune one cavity.

In one embodiment of `my invention, the selftuning klystron is employedas an amplifier. This consists in using a broad beam responsive bunchercavity with a high Q catcher cavity, the catcher cavity containing aself-tuning probe. This cause the resonance frequency of ouramplier tovary in response to the frequency of the input oscillations.

The novel features that I consider characteristic of my inventionare setforth with particularity in the appended claims. The invention itself,however, both as to its organization land its method gfQperaiogwgetherfwith aqaitionai objects and advantages thereof will bestbe read in connection with the accompanying drawing; in which, thesingle figure is a schematic showing of apparatus employed in oneembodiment of my invention.

Referring in detail to the drawing, a heated cathode 2 is employed as asource of electrons. Near the cathode are the biasing grid 4, focusingone or more grids 251ahd the accelerating anode 26 which serves toproduce an electron beam.

In line with the cathode and focusing electrodes.

and 26 is a buncher cavity 6. Connected to. the buncher cavity by ametal connection tube 8 is the catcher cavity l0.- Beyondthe catchercavity is the accelerating plate.|2. Thecathode 2, -acceleratingelectrodes 25 and 2S,- bunchercavity 6, catcher cavity I0 and anodeI2.are sub.- stantially in line so that the electrons leaving thecathode 2 will move directly toward the plate I2. There are openings I4in the catcher cavity I and the buncher cavityf to allow the. passage ofthe electrons through these cavities. Inserted. in the side of: thecatcher. cavity and the buncher cavity. are adjustable probes I5, I8 forvarying the impedance of the cavities and. thus varying theiry resonantfrequency. These probesv I6, I8 are. preferably manually adjustable.

If the apparatus is to be employed as an oscillator,v the catcher.cavity ID and the buncher cavity Ewill be connected electrically,preferably by a cofaxial cablev 20.

Inside thecatcher cavity is a metal responsive cap or probe 2.2connected by a spring 24 to the connecting tube 8 between the cavities.This responsiver probe 22.. will move in response to a change intheelectric. force produced by the oscillating field, within. thev cavityvcausing the catcher cavity to be self-tuning,

The electric force per unit area measured in gms-Jem.2 which is found toexist in an electric 'where E, the electric field strength at theresponsive probe 22. is `measured in volts/cm.

The magnitude of the electric field existing in the considered type ofhigh Q, high power cavity is in the order of. 106 volts/cm. when, as anexample, pulsed magnetrons are used. With a vduty cycle of,.say 2microseconds at a repetition period of l millisecond, the average forceper unit probe area is then in the order of 1 g./cm.2.

Another equation to describe the electrical force on the probe isG=9.041i l01g./cm.2

where P is the power level in wattswhich has been built up in thecavity, R the shuntrresistance of the cavity in ohms and d the spacingin cm. in which the field acting on the probe is confined. Again usingan example, if. we assume a typical klystron which. generates 500 Wattsof power in a cavity of 1 cm. gap spacing and 2 million ohm shuntresistance, an electrical force on the probe in the. order of 1 .g./cm.2is obtained.

In either case, then, it will be evident that enoughmechanical force isgenerated within a cavity operating. at conventional power levels toactuate a probe for a distance of a fewhundredths of amillimeter.Calculations show that a probe displacement equal to one hundredthof oneper cent lof the length of a typical cavity would change theresonantfrequency of a cavity having a resonant frequency of 3000"megacycles -by an amount of the order of; one'megacycle, in-

dicating that the orders of magnitude of the effects involved in thesystem are sufficient to produce the results desired.

The responsive probe 22 is normally retained in a withdrawn positionagainst the struture by means of the helical spring 24 designed to havea predetermined restoring force exerted on the probe against the forceestablished by the electric field E which tends to move the probe awayfrom the structure in opposition to the force exerted by the restoringspring independent of the direction which the alternating field mighthave at a given moment. When the alternating eld changes in strength",the' forcev on the probe changes, thus causing the probe to move.

In my oopendingapplication Serial No. 65,963, led December 17 1948, itis shown that as the frequency of the cavity changes, the probe willmove in such a direction as to cause the resonant frequency of thecavity to approach the new frequency impressed on the cavity. The.responsive probe will adjust itself to anew position, such that arelatively great reduction of frequency lof the cavity will have takenplace without material change in the amplitude of the oscillationswithin thev cavity.

In order to initiatean'operation of the'present system, it is essentialthat the generated fre.- quency be started at avalue for which thecatcher cavity IIl is resonant inthe restposition ofthe responsive probe22 andthereafter varied gradu.- ally to a final desired value, theresponsive probe acting continually to retune the cavity andv tomaintain correspondence between thefcavity fre.- quency and the drivingfrequency. This may also be accomplishedby setting thedriving fre.-quency at a pre-established value and moving the responsive probe to asuitable position to establish frequency correspondence betweenthefrequency of this source andthe tuning of the cavity. 'I'he movement ofthe responsive probe may be produced by meansl of a magnet or manuallyfrom outside ofthe cavity.

The present systemyserves not only to adjust the frequency of a cavity,but also to keep the power level of the cavity constantv independent ofvariations of generated power beyond the adjusted value,nsince anincrease or adecerasezin input power level acts. onthe responsive. probeprecisely as doesa variation in frequency, by es.- tablishing a modifiedforce on the responsive probe, which serves4 to vary the displacementthereof, whereby a new cavity resonance fre-- quency is established,which serves to re-establish substantially the original amplitude ofthe'. field E which continues. to` alternate. with' the signalfrequency.

In one embodiment of my invention I employ a self-,tuning klystron. asan oscillator. The catcher cavity', I0 is preferably a: high Q cavity,but the buncher cavity 6.1may,.at the expenseof gain, have broadbandyresponse. To start' the osci11ator,. thev adjustable tuning. probe I6.'in the bunchercavity is adjusted gradually towards lower frequenciesybeginning. with. a frequency known tobe abovelthe. resonantfrequency.cor@ responding .to thefrestv position ofthe responsive tuning probe.A's the adjustabletuning` probe in the b'uncher cavity isA changed,the-frequency of the-buncher cavity will also'change. When the frequencyof the buncherA cavity reaches a frequency corresponding to theresonant*v frequency at the rest-position of the responsivetuning probe,the klystron will begin` to oscillate: By further reducingthefrequency-of the'bunclier cavity by adjusting the adjustable tuningprobe, the responsive probe will be pulled into the field, thusmaintaining the natural characteristics of the catcher cavity nearresonance and at a constant power level. Adjustment of the adjustabletuning probe in the buncher cavity is continued until the desiredfrequency is reached.

I have thus avoided the difficulty of adjusting two cavitiessimultaneously to the same frequency. Instead, the catcher cavity inspite of its high Q and high power level, simply follows the bunchercavity to the desired frequency. The catcher cavity of the klystron willcontinue to maintain its resonant frequency near that of the bunchercavity even under detuning influences, such as load or temperaturechanges.

In another embodiment of my invention, the self-tuning klystrons may beemployed as an amplifier. In the amplifier it is desirable to employ abroad band responsive buncher cavity with a high Q catcher cavity aswith the oscillator. Here, however, the tuning of the klystron isproduced by moving the adjustable tuning probe I8 in the catcher cavityuntil resonance occurs in the rest position of the responsive probe 22,while the signal to be amplified is being applied to the klystrons.

When the adjustable tuning probe I8 reaches the position where resonanceoccurs in the catcher cavity with the responsive probe 22 in the restposition, the responsive probe 22 will react to the forces applied bythe electrical field and will continue to respond to adjustments of theadjustable tuning probe I8 so as to maintain the resonance frequency ofthe catcher cavity near the input frequency.

From what has been said so far, it is seen that it is desirable that therest position of the responsive probe 22 correspond to a resonantfrequency above that frequency which it is desired to operate theklystron. Thus, the responsive probe, in moving to that positioncorresponding to lower frequencies than the rest position, will be ableto adjust itself to changes in input frequency and power.

Since numerous changes may be made in the above-described constructionand different embodiments of the invention may be made without departingfrom the spirit and scope thereof, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

I claim as my invention:

1. A high-frequency oscillation generator comprising an envelope havingtherein a buncher cavity resonator, and a catcher cavity resonator,

Cil

said catcher cavity resonator containing a metal cap, a resilientmounting for said cap connecting said cap to a wall of said catchercavity, said mounting having an elasticity of a magnitude to allow thelocation of said cap to move in response to changes in the forceexterted on it by the oscillations in the catcher cavity resonator so asto tune said catcher cavity resonator, said resilient mounting being theonly support for said cap.

2. A high-frequency amplifier comprising an envelope having therein asource of electrons, a buncher cavity resonator, a catcher cavityresonator, and an anode, said catcher cavity resonator containing ametal cap, a resilient mounting for said cap connecting said cap to awall of said cavity so as to allow the location of said cap to change inresponse to a change in the force exerted on it by the oscillations inthe catcher cavity resonator, said resilient mounting being the onlysupport for said cap.

3. A generator for electromagnetic oscillations comprising: a source ofelectrons, acceleration electrodes, a buncher cavity resonator, acatcher cavity resonator, an anode, sai-d catcher cavity resonatorcontaining a re-entrant projection, a metal cap substantially coveringan end of said projection, a spring attached to said metal cap, saidspring constituting the sole support for said metal cap, said springhaving a strength such that said cap will move in response to a changein said eld force.

4. A generator for electromagnetic oscillations comprising: a source ofelectrons, acceleration electrodes, a buncher cavity resonator, acatcher cavity resonator, an anode, said catcher cavity resonatorcontaining a re-entrant projection, a. metal cap substantially coveringan end of said projection, spring means attached to said metal cap, saidspring means constituting the sole support of said metal cap, and saidspring means having a strength such that said cap will move in responseto a change in said field force.

MAX GARBUNY.

References Cited in the ille of this patent UNITED STATES PATENTS NumberName Date 1,605,911 Banneitz Nov. 9, 1926 2,027,751 Nelson Jan. 14, 19362,311,658 Hansen et al Feb. 23, 1943 2,374,810 Fremlin May 1, 19452,388,289 Ronci Nov. 6, 1945 2,492,996 Haxby Jan. 3, 1950 2,501,152Becker Mar, 21, 1950 2,531,214 Harris et al. Nov. 21, 1950

